Ferrous base coated with zinc and chromium



Jun-e 6, 1967 J, A NELSON ET AL FERRoUs BASE COATED WITH ZINC AND CHROMIUM Filed Nov. 29, 1963 ON m;

Invenio'r JohnAJelOrz Fred E. Wills 0 1; by u ogs 0.1.1 0.9

Morne@ atented June 6, 1967 3,323,881 FERRUS BASE COATED WITH ZINC AND CHROMIUM John A. Nelson, Tucson, Ariz., and Fred E. Wills, Gary, Ind., assignors to Inland Steel Company, Chicago, Ill., a corporation of Delaware Filed Nov. 29, 1963, Ser. No. 326,809 6 Claims. (Cl. 29-1S3.5)

The present invention relates generally to the production of a ferrous metal sheet vand strip material having a protective metal coating thereon, and more particularly to a galvanize-d steel sheet or strip material having irnproved resistance to corrosion.

The increased competition olfered to galvanized steel sheet and strip products by other materials of construction makes it increasingly desirable to provide galvanized steel sheet and strip products having improved resistance to corrosion which can be produced at a cost competitive with other materials of construction, such as aluminum or plastic coated sheet materials. And, since an appreciable quantity of galvanized strip and sheet material is used by forming or drawing, it is highly desirable that the galvanized sheet and strip material be capable of being formed or drawn without adversely affecting the corrosion resistance thereof.

It is therefore an object of the present invention to provide ferrous metal strip and sheet products having improved corrosion resistance.

It is also an object of the present invention to provide a ferrous metal strip having improved corrosion resistance by treating a ferrous metal strip in-line with a continuous metal coating line.

It is a further object of the present invention to provide markedly improved corrosion resistant galvanized steel strip and sheet products which can be produced continuously in-line with a continuous galvanizing process line.

It is another object of the present invention to provide galvanized steel strip vand sheet materials which have imparted thereto markedly improved corrosion resistance without having the general utility of the galvanized materials impaired.

It is still another object of the present invention to provide galvanized steel strip and sheet products having greatly improved corrosion resistance which are capable of being drawn or formed without impairing the corrosion resistance thereof.

Other objects of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawing and claims to follow.

Heretofore it has been thought that very thin chromium coatings applied directly over zinc on a steel base would have no beneficial effect, since very thin coatings of chromium were not thought capable of adhering to :a galvanized surface and forming an impervious coating directly thereover. It has also been found that very thin chromium films having a thickness of .00002 inch when applied directly on the ferrous metal sur-face of a steel strip do not improve the salt spray corrosion resistance of the steel strip.

It has now been discovered, however, that the corrosion resistance of a galvanized conventional low carbon steel strip or sheet, such as produced on a continuous hotn dip or electrolytic galvauizing line, can be greatly improved with regards to its corrosion resistance by applying continuously to the as plated or temper rolled zinc coating as it leaves the continuous galvanizing line a very thin lm of electrolytically deposited chromium having a thickness range from yat least about .000001 inch but less than .00002 inch with a preferred thickness being from about .000004 inch to about .00001 inch. An extremely thin chromium coating having the foregoing thickness range can be readily applied to a galvanized steel strip electrolytically within the time and space limitations imposed by a conventional in-line continuous galvanizing process line yas the strip leaves the galvanizing line and while traveling at its normal rate. Thus, it is possible for the first time to produce a galvanized steel strip or sheet product having very substantially improved corrosion resistance ina more economical manner than heretofore. And, because the chromium lm is extremely thin, the chromium coated galvanized steel strip or sheet can be formed or drawn in the same manner as the uncoated galvanized strip or sheet without destroying the improved corrosion resistance which has been imparted thereto.

More particularly, in accordance with the present invention, a galvanized steel strip of any conventional type, such as a low carbon steel strip having a thin coating of zinc thereon, preferably between about .0001 inch to .002 inch in thickness, exhibits substantially improved corrosion resistance when a thin iilm of chromium having a thickness of as little as .000001 inch is electrolytically deposited directly on the surface of the'zinc coating. In the accompanying drawing, the graph showing the rela tionsl'iip between the electrolytically applied chromium film thickness and the corrosion ratio 1 of a chromium coated conventionally hot dip galvanized steel strip having on each side thereof a zinc coating thickness of about .0005 inch indicates that there is a very substantial increase in the corrosion resistance of a galvanized steel strip when a film of chromium having a thickness of only .000001 inch is provided directly on the galvanized surface and that the corrosion resistance continues to increase at a fairly rapid rate as the thickness of the chromium film is increased to about .000004 inch. There does not :appear to be any increase in corrosion resistance of the galvanized strip as the continuously electroplated chromium film is increased in thickness above .00001 inch to about .00002 inch. The failure to obtain any increase in corrosion resistance with the thicker chromium film may be due to the Vformation of cracks therein. Actually, increasing the coating thickness of the chromium lm from .000004 inch to .00001 inch only increases the corrosion ratio from about 1.9 to 2.0, land this slight increase amounting to 5% is obtained by a 2 5-fold increase in the chromium coating thickness. From a commercial viewpoint it would appear inadvisable and unnecessary to employ a chromium coating having a thickness in excess of about .00001 inch and probably not above .000004 or .000005 inch.

The improvement in corrosion resistance is effected by applying an extremely thin film of chromium of the herein disclosed type to any of the conventional galvanized steel sheet or strip materials, including both conventional hot dip galvanizing and electrolytic galvanizing steel sheet and strip materials. In the following Table I yare shown the test results achieved by chromium plating in accordance with the present invention two conventional steel mill produced hot dip galvanized steel products of the indicated types:

1 The corrosion ratio value:

Time to failure of sample Avg. time to failure of controls The failure of a sample or control is indicated by the first appearance of red rust thereon. If the corrosion ratio value is greater than 1.0, the resistance to corrosion is improved; if less than 1.0, the corrosion resistance is impaired. A corrosion ratio of 2.0, for example, indicates an increase in corrosion resistance of over the control.

TABLE I EFFECT OF CHROMIUM PLATING ON THE 5% SALT SPRAY TEST PERFORMANCES OF MILL PRODUCED GALVANIZED STEELS Corrosion Ratios It will be evident from Table I that the corrosion resistance of conventional galvanized sheet material of the type normally produced on :a standard continuous galvanizing 119 and havmg the SPelled thm Chromlllm fh'l alphed 15 resistance of the hot dip galvanized and electrolytic zinc dlrectly over a Standard Zin? Spelter hot dlp ,:Oatmg ,1S not coated steel materials. The salt spray test performances affected by the temper roumg of the galvamzed stnp be' of these chromium lated stri s hot di coated lW'th the fore chromium plating, since the corrosion ratios of the 11 d p 1 p p05 1 respective temper rolled (Le. extra smooth) materials spe'la ea ree Zuic Spe ler contammg magneslum were of the same general magnitude as when not temper havlng a z1nc-coatmg weight of only .40 ounce per-ft2 rolled before chromium plating. These chromium coated and a Chromium mm thlckness 0f only -000002 mch strips can be drawn and formed without destroying their (-002 m11) ranged from about 1200 t0 2000 hours, and improved `Corrosion resigning@ these are approximatelyY four to six times better than that K In the following Table II are shown the results achieved 0f regular hot dP galvanized Steel Strips having a heavier when hot dip galvanized sheet material and electrogalvazinc coating weight but without the extremely thin chronized sheet materials of the specied type are subjected mium surface film. t0 a Standard Continuous 5% Salt Spray 'fest for COTTOSOD In the tests of the electrogalvanized chromium coated resistance aftf Coating WT-h a hfl lm 0f Chromium in samples reported in Table Il, the corrosion ratios obtained accordance Wlth the present inventlon: were extremely high, approximately 4.2, for a chromium TABLE H--EFFECT 0F CHROMIUM COATING ON THE coating thickness of .000005 inch to .000007 inch. This is llyi'jgFstiinpDpdTunflRiiisrs OF VARIOUS twice as high as has been obtained with similarly chromi- AHOT Dip GALVANIZED COATINGl um COated hot dip galvanized Steel samples. Moreover, t an electrolytic zinc coating weight of only .25 oz. per Zn Coating Chromium Corrosion, Number square foot per side was used to obtain these very high Weight/Side 's Ratio gurlgt corrosion ratios. Thus, even with very thin electrolytically applied Zinc coatings, applying a chromium film increases (None) Control so@ the corrosion ratio of the electrogalvanized strip about ata?.-

2222 .o four me .002 m11 2.0 1,200 The thm chromium surface films employed 1n the present invention can be applied electrolytic-ally by any con- B ELECTROGALVAN1ZED COATINGz ventional chromium plating procedure and by any conventional chromium plating bath to produce a bright chro- .25 oz./sq.ft .005 mi1 4.2 1, 310 45 mium surface lm. The composition of several aqueous gjgj- 'gglglj Conr'ol 1 plating baths, including special high speed plating solutions, and the operating conditions which can be employed ed a .05% Mg., Lead-free Zine Spelter.

2 Plated from Standar TABLE IIL- PLATING C O It is evident from Table Il that an extremely thin chromium surface film substantially increases the corrosion are shown in the following Table Ill;

NDITIONS FOR VARIOUS CHROMIUM PLATING BATHS *Produced by Metal Thermit Corporation.

In the following Table IV there is shown a comparison of the corrosion resistance of a conventional hot dip galvanized steel strip product having a uniform coating Weight of about .603 oz./ft.2 per side (i.e. coating thickness per side of about .0005 inch) coated by one of the above indicated chromium plating baths under the conditions specified:

TABLE ivf-COMPARISON OF VARIOUS CHROMIUM PLATING BATHS- 5% SALT FOG TEST RESULTS Current Plating Samples Corro- Chromium Plating Bath Time, Density, Temp., Thick- Failure, sion See. Amps per F. ness, Hours to Ratio ft.2 mils Red Rust Normal 10 120 130 001 672 1. 72 10 120 130 001 648 1. 66 60 120 130 005 696 1. 78 60 120 130 005 816 2. l0 Boruhauser 10 300 65 001 480 1. 23 10 300 65 002 792 2. 02 60 300 65 003 696 1. 78 60 300 65 004 840 2. 15 Unimet CR 110* 10 340 130 004 480 1. 23 10 340 130 004 576 1. 47 60 340 130 020 672 1. 72 60 340 130 020 696 1. 78 Unimet CF 520* 10 240 120 003 792 2.02 10 240 120 003 648 1. 66 60 240 120 010 840 2. 15 60 240 120 020 746 1. 91

* Produced by Metal Thermit Corporation.

Since there is no marked advantage resulting from using any of the special chromium plating baths, it is preferred to apply the thin chromium lm by means of a normal chromium plating both of the type disclosed in Table III, particularly since the latter is more easily controlled than any of the other chromium plating baths which have .been tested.

In applying the extremely thin chromium film to the surface of a zinc coating on a steel strip the usual good operating practice should be followed and the zinc surface when chromium plated should be free of surface oxides and contamination. A bright chromium surface film can be plated from a normal chromium acid sulfate plating bath without benefit of any special pre-treatment when a clean oxide-free zinc surface is presented. If desired, however, a standard bright dip can be applied to the zinc coated strip immediately before electroplating. An electrolytic pre-treatment which results in forming an oxide film on the zinc coating should be avoided where a bright appearing final product is necessary. If desired, the resultant chromium plated galvanized steel strip can be further treated to provide satisfactory Wet storage stain resistance without impairing its corrosion resistance by subjecting the plated strip to a conventional chromate treatment, such -as a three second treatment in about a .3% ammonium chromate solution.

As previously indicated, the present invention can be readily incorporated into a conventional continuous galvanizing line, since the extremely thin chromium ilm required in the present invention can ybe electrolytically applied by means of conventional electropl-ating equipment within a matter of seconds and therefore does not require extensive space which would make the chromium plating treatment entirely impractical for continuous inline processing with a continuous galvanizing line. For example, a commercial continuous galvanizing line treating a 48-inch wide low carbon steel strip normally runs at a rate of 100 ft./min. for 20 gauge steel strip material, and at a rate of 150 ft./min. for 24 gauge material when operating under normal conditions. If We assume that a galvanizing line is producing 48-inch Width material hav'mg a 24 gauge thickness and traveling at a rate of 150 ft./min., the amperage required to apply a .00001 inch chromium coating on both sides lat a current efficiency of is about 200,000 amperes. The plating feet. This length of plating section gives a plating time for the above product of approximately 33 seconds. If a chromium iilm having a thickness at the lower end of the preferred range of chromium coating thickness is used, the length of the foregoing plating section is reduced to approximately 43 feet when chromium plating under the same operating conditions. If should also be understood that, while possible, it is considerably more expensive to provide a chromium film on the galvanized steel strips or galvanized cut sheet material in accordance with the present invention after the continuous strip has been removed from the continuous galvanizing line and furthermore introduces additional operating problems which are entirely avoided by the preferred in-line treatment.

In the foregoing description, data and examples illustrating the present invention wherein zinc coatings are referred to and used, it should be understood that the zinc coating can be composed of zinc or any conventional or special zinc alloy formulation. And, whereas the specific embodiments of the present invention have used zinc for the protective metal coating on the ferrous metal strip base material, other protective metals, such as cadmium, aluminum, or alloys thereof, can be used in place of zinc and come within the broad scope of the present invention.

We claim:

1. A ferrous metal product comprising a ferrous metal strip with a protective metal coating containing zinc on at least one surface thereof with said coating having electrolytically applied directly thereon a thin surface film of metallic chromium having a substantially uniform thickness ranging between at least about .000001 inch and less than .00002 inch, whereby said galvanized steel product is provided with a substantially improved corrosion resistance.

2. A steel product as in claim 1, wherein said zinc coating has a thickness of about .0001 inch to .002 inch.

3. A steel product as in claim 1, wherein said zinc coating is a hot dip galvanized zinc coating consisting essentially of a lead-free zinc spelter containing about .05% magnesium.

4. A steel product as in claim 1, wherein said zinc coating is an electrolytically deposited layer of zinc.

5.7A steelproduct as in claim 4 wherein said zinc coating has a thickness of about .25 oz./sq. ft. of surface area.

6. A steel product having a protective metal coating comprised essentially of4 zinc exhibiting substantially improved corrosion resistance and good drawing properties which comprises; a steel strip having a .protective metal coating comprised essentially of zinc on at least one surface of said strip, and said protective coating having a film of metallic chromium electrolytically applied directly on said coating with said film having a substantially uniform thickness between about .000004 inch and .00001 inch.

References Cited UNITED STATES PATENTS Humphries 29-196.6 Peirce et al 75-178 Fiedler.

Harris.

Du Rose 29-196.6 X Kenmore 204-55 Kennedy 29-65 Miller 209-41 X HYLAND BIZOT, Primary Examiner. 

1. A FERROUS METAL PRODUCT COMPRISING A FERROUS METAL STRIP WITH A PROTECTIVE METAL COATING CONTAINING ZINC ON AT LEAST ONE SURFACE THEREOF WITH SAID COATING HAVING ELECTROLYTICALLY APPLIED DIRECTLY THEREON A THIN SURFACE FILM OF METALLIC CHROMIUM HAVING A SUBSTANTIALLY UNIFORM THICKNESS RANGING BETWEEN AT LEAST ABOUT .0000001 INCH AND LESS THAN .00002 INCH, WHEREBY SAID GALVANIZED STEEL PRODUCT IS PROVIDED WITH A SUBSTANTIALLY IMPROVED CORROSION RESISTANCE. 